In medical engineering, for example, image-assisted interventional methods have long played an important role. This applies particularly to surgical interventions on a patient where surgical instruments such as laparoscopes, endoscopes, needle robots, etc. are controlled via various image-assisted methods.
Thus in the simplest case, purely image-assisted positioning of medical instruments is performed, i.e. instrument positioning takes place by direct visual display, i.e. is carried out and monitored on the basis of the images. Examples of this are needle guidance e.g. for biopsies or HF ablations by means of ultrasound, CT-fluoroscopy or x-ray fluoroscopy. These methods are characterized by real-time capability and are currently standard in many fields of application.
Another widely practiced method in medical engineering is the use of surgical navigation based on pre-operative images. In this case instrument positioning is performed with the aid of navigation systems using patient image data acquired prior to the actual operation. This image data is generally based on CT or MR images, but SPECT (Single Photon Emission Computed Tomography) or PET (Positron Emission Tomography) images are also being increasingly used. Operating methods based on pre-operative images are used in orthopedic robotics. Examples of robot-assisted surgical interventions are knee and hip operations. In the prior art, such interventions are performed exclusively using CT x-ray images taken of the area to be operated on prior to the surgical intervention.
However, other interventions in which changes of position occur or may occur may require continuous control images during the intervention in order to ensure safe positioning e.g. of the medical instruments and therefore safe performance of the operation. In this case image data sets are additionally combined with one another during the intervention by if necessary recording new images of the patient during the operation and inserting both into existing 3D image data sets. The use of such a medical method for assisting surgical interventions on a patient is also particularly called for when the surgeon's view of the patient end of a medical instrument guided by him which is inserted in the patient's body is obstructed and/or a position and shape of a body part or organ shown in the previously recorded image does not coincide with the actual position and shape of the body part or organ during the operation.
The abovementioned problem of change of position becomes even more acute when not only one-off position shifts after pre-operative image recordings are involved, but when moving organs must be recorded using imaging methods. This basically relates to the heart with its blood-supplying coronary arteries, heart beat and respiration generally being responsible for such movements.
Every contraction of the cardiac muscle and therefore every pumping function of the heart is preceded by an electrical stimulus. This electrical potential variation across the heart can be picked up on the surface of the body. The movement of the heart caused by contraction of the cardiac muscle therefore occurs periodically and essentially in synchronism with the so-called surface ECG (electrocardiogram). Therefore, sections of the ECG can be assigned to particular periodically recurring, identical phases of cardiac movement. This fact is already used in many examination techniques using imaging methods and is therefore prior art. Examples include cardiac CT or electroanatomical mapping with the Carto system. Systems of this kind enable the user to control image acquisition or the recording of electroanatomical maps of the patient's heart using and as a function of the ECG. The EGG produces a trigger signal normally at the instant of the R-spike. However, this trigger signal therefore unfortunately occurs in the strongest phase of cardiac movement and is therefore unsuitable for image acquisition as, in order to minimize motion artifacts, image data should be recorded in as quiet a movement phase of the heart as possible. In simple form, the acquisition of image data is therefore shifted manually with respect to a characteristic point of the ECG by means of an adjuster. This produces image data (generally 3D image data) recorded in a particular phase of cardiac movement.
As mentioned in the introduction, interventional procedures frequently involve overlaying this pre-operatively recorded 3D image data with 2D image data, thereby enabling the physician performing the intervention to be supplied with additional information from the 3D image data set during the intervention. In another application the physician is given current control images from the 2D image data set, or the position and attitude of medical instruments are to be monitored. For this purpose the 3D image data set must be synchronized with the 2D image data set. Ideally a 2D image data set should therefore be overlaid with the a pre-operatively recorded 3D image data set of the same phase of cardiac movement. For this purpose information is first required as to the cardiac phase in which the 3D image data was generated. The examiner should therefore ideally also be offered a shift of the trigger pulse for the acquisition of the 2D image data. This information is normally specified as a percentage of a period, i.e. 100% being a complete cardiac cycle from R-spike to R-spike. In another form the interval with respect to the R-spike is specified in milliseconds.
In so far as the information about this is available to the examiner, he must control the acquisition of the 2D image data in the same way as for acquisition of the pre-operative 3D image data. As this method must first be disadvantageously preceded by viewing and evaluating the pre-operative image data, this is a very time-consuming and generally also error-prone process. Errors may, however, result in the current image recordings either being poorly synchronized with respect to the pre-operative images and therefore of limited use or in them having to be repeated in their entirety, which in turn results in increased patient dose load.
US 2005/0137661A1 discloses a method and a system using said method for treating cardiac arrhythmias by means of catheter ablation, wherein the CT scanner images of the heart triggered by a patient ECG are reconstructed to produce 3D image data and made available to a central unit of an x-ray machine. The current “live” ECG is likewise recorded and synchronizes the 3D images with the patient's cardiac cycle by detecting the ECG time stamp on the 3D images and comparing the ECG with the corresponding values of the current “live” ECG. In this way the 3D images are therefore synchronized with continuously recorded x-ray images. In a subsequent step a 4D image data set in whose formation only the 2D image data in synchronism with the 3D image data set is involved is then reconstructed and displayed to the interventional team. Although only selected, namely synchronous image data from the same phase of cardiac movement as the pre-operative 3D image data set is involved for reconstructing the 4D image data set produced here, the patient is disadvantageously exposed to a higher radiation dose than is absolutely necessary. In addition, the image capture device itself is caused to make a series of recordings that do not necessarily need to be made.